The space station's Russian commander, Dmitry Kondratyev, said the human dream of flying to the stars came true with Soviet cosmonaut Yuri Gagarin's flight on April 12, 1961. Twenty-three days later, American Alan Shepard became the second man in space.

"Fifty years is a short period of time in history, but look at that leap from a small spacecraft to the huge International Space Station," Kondratyev told the AP. "We hope that during the next 50 years, another leap that is not less than has been done, will be done."

As for the shuttle anniversary, Garan said Columbia's launch on April 12, 1981, inspired him to become an astronaut. He yearned to be an astronaut as a young child, but the dream went away until Columbia blasted off on the first shuttle flight.

The very next day, he went to his college advisers and signed up for a heavy load of math and science courses.

"That dream was reawakened in me," Garan said. "For me, that's a pretty strong evidence of the importance of the space program, toward education and inspiring young people to study math and science."

As a two-time passenger aboard Columbia — which was destroyed during re-entry in 2003 — space station resident Catherine Coleman said it's a special day for her as well.

Twenty-one museums and science and visitor centers are in hot competition.

The space station crew declined to pick a favorite.

"I grew up all over the country," Coleman told the AP, "and so I actually have ties, I think, to every one of the cities that are targeted. ... I'm trying to be impartial."

Only two shuttle missions remain, both to the space station. Endeavour is due to blast off April 29, and Atlantis on June 28. Discovery, the oldest surviving shuttle, ended its flying career last month.

The space station astronauts will get Tuesday off as a special holiday. They will hold a news conference and link up with celebrations in Moscow, then enjoy a celebratory feast Tuesday evening.